U.S. patent number 10,003,072 [Application Number 15/101,792] was granted by the patent office on 2018-06-19 for positive electrode active material for secondary battery, method for producing same and secondary battery.
This patent grant is currently assigned to NEC Corporation. The grantee listed for this patent is NEC Corporation. Invention is credited to Takehiro Noguchi, Shin Serizawa, Makiko Takahashi.
United States Patent |
10,003,072 |
Noguchi , et al. |
June 19, 2018 |
Positive electrode active material for secondary battery, method
for producing same and secondary battery
Abstract
A secondary battery having an improved life characteristics is
provided by the use of a positive electrode active material for a
secondary battery, comprising (a) a surface layer comprising a
lithium metal composite oxide having a spinel crystal structure
represented by space group Fd-3m, and (b) an internal portion
comprising a lithium metal composite oxide having a spinel crystal
structure represented by space group P4.sub.332.
Inventors: |
Noguchi; Takehiro (Tokyo,
JP), Takahashi; Makiko (Tokyo, JP),
Serizawa; Shin (Tokyo, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
NEC Corporation |
Tokyo |
N/A |
JP |
|
|
Assignee: |
NEC Corporation (Tokyo,
JP)
|
Family
ID: |
53273254 |
Appl.
No.: |
15/101,792 |
Filed: |
November 5, 2014 |
PCT
Filed: |
November 05, 2014 |
PCT No.: |
PCT/JP2014/079370 |
371(c)(1),(2),(4) Date: |
June 03, 2016 |
PCT
Pub. No.: |
WO2015/083481 |
PCT
Pub. Date: |
June 11, 2015 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20160308199 A1 |
Oct 20, 2016 |
|
Foreign Application Priority Data
|
|
|
|
|
Dec 4, 2013 [JP] |
|
|
2013-250861 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C01G
53/54 (20130101); H01M 4/502 (20130101); H01M
4/525 (20130101); H01M 10/0525 (20130101); H01M
4/366 (20130101); H01M 4/523 (20130101); H01M
4/505 (20130101); C01G 45/1242 (20130101); H01M
4/0471 (20130101); H01M 4/131 (20130101); C01P
2004/84 (20130101); C01P 2002/85 (20130101); C01P
2006/40 (20130101); C01P 2002/76 (20130101); Y02E
60/10 (20130101) |
Current International
Class: |
H01M
4/36 (20060101); C01G 53/00 (20060101); H01M
4/131 (20100101); H01M 4/525 (20100101); H01M
4/505 (20100101); C01G 45/12 (20060101); H01M
4/04 (20060101); H01M 10/0525 (20100101); H01M
4/52 (20100101); H01M 4/50 (20100101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
11-339805 |
|
Dec 1999 |
|
JP |
|
2002-158007 |
|
May 2002 |
|
JP |
|
2009-80979 |
|
Apr 2009 |
|
JP |
|
4683527 |
|
May 2011 |
|
JP |
|
4770113 |
|
Sep 2011 |
|
JP |
|
2012-190648 |
|
Oct 2012 |
|
JP |
|
2013-89389 |
|
May 2013 |
|
JP |
|
2013-93167 |
|
May 2013 |
|
JP |
|
2013-93170 |
|
May 2013 |
|
JP |
|
2014-110176 |
|
Jun 2014 |
|
JP |
|
Other References
H-M. Cho et al., "Effect of Ni/Mn Ordering on Elementary
Polarizations of LiNi.sub.0.5Mn.sub.1.5O.sub.4 Spinel and Its
Nanostructured Electrode", Journal of the Electrochemical Society,
160 (9) A1482-A1488, 2013. cited by applicant .
J.-H. Kim et al., "Effect of Ti Substitution for Mn on the
Structure of LiNi.sub.0.5Mn.sub.1.5--xTi.sub.xO.sub.4 and Their
Electrochemical Properties as Lithium Insertion Material", Journal
of the Electrochemical Society, 151 (11) A1911-A1918, 2004. cited
by applicant .
L. Guoqiang, LiNi.sub.0.5Mn.sub.1.5O.sub.4 Spinel and Its
Derivatives as Cathodes for Li-Ion Batteries, Lithium Ion
Vatteries-New Developments, In Tech, pp. 83-100, Feb. 2012. cited
by applicant .
International Search Report and Written Opinion dated Jan. 13,
2015, in corresponding PCT International Application. cited by
applicant.
|
Primary Examiner: Chmielecki; Scott J
Attorney, Agent or Firm: Finnegan, Henderson, Farabow,
Garrett & Dunner, L.L.P.
Claims
The invention claimed is:
1. A positive electrode active material for a secondary battery,
comprising: (a) a surface layer comprising a lithium metal
composite oxide having a spinel crystal structure represented by
space group Fd-3m, and (b) an internal portion comprising a lithium
metal composite oxide having a spinel crystal structure represented
by space group P4.sub.332.
2. The positive electrode active material for a secondary battery
according to claim 1, wherein (a) the surface layer comprises a
lithium metal composite oxide having a crystal structure
represented by space group Fd-3m and represented by formula (1):
Li.sub.a1(M1.sub.x1Mn.sub.2-x1-y1Y1.sub.y1)O.sub.4 (1) wherein,
0.4.ltoreq.x1.ltoreq.1.2, 0y1.ltoreq.1, x1+y1<2,
0.ltoreq.a1.ltoreq.1.2; M1 comprises at least one selected from the
group consisting of Co, Ni, Fe, Cr and Cu; Y1 is at least one
selected from the group consisting of Li, B, Na, Al, Mg, Ti, Si, K
and Ca; and (b) the internal portion comprises a lithium metal
composite oxide having a crystal structure represented by space
group P4.sub.332 and represented by formula (2):
Li.sub.a2(M2.sub.x2Mn.sub.2-x2-y2Y2.sub.y2)O.sub.4 (2) wherein,
0.4.ltoreq.x2.ltoreq.1.2, 0.ltoreq.y2.ltoreq.1, x2+y2<2,
0.ltoreq.a2.ltoreq.1.2; and M2 comprises at least one selected from
the group consisting of Co, Ni, Fe, Cr and Cu; Y2 is at least one
selected from the group consisting of Li, B, Na, Al, Mg, Ti, Si, K
and Ca.
3. The positive electrode active material for a secondary battery
according to claim 1, wherein the surface layer having a crystal
structure of Fd-3m has a thickness in a range of 1 nm of more and 1
.mu.m or less.
4. The positive electrode active material for a secondary battery
according to claim 2, wherein in the general formula (1), M1
comprises at least Ni.
5. The positive electrode active material for a secondary battery
according to claim 2, wherein in the general formula (2), M2
comprises at least Ni.
6. The positive electrode active material for a secondary battery
according to claim 2, wherein in the general formula (1),
0.45<x1<0.8 is satisfied.
7. The positive electrode active material for a secondary battery
according to claim 2, wherein in the general formula (2),
0.45<x2<0.55 is satisfied.
8. The positive electrode active material for a secondary battery
according to claim 2, wherein in the general formula (1),
0.ltoreq.y1.ltoreq.0.3 is satisfied.
9. The positive electrode active material for a secondary battery
according to claim 2, wherein in the general formula (2),
0.ltoreq.y2.ltoreq.0.3 is satisfied.
10. A positive electrode comprising the positive electrode active
material for a secondary battery according to claim 1.
11. A secondary battery comprising the positive electrode according
to claim 10.
12. A process for producing a positive electrode active material
for a secondary battery, comprising the steps of: (A) forming a
particle comprising a lithium metal composite oxide having a spinel
crystal structure represented by space group of P4.sub.332; and (B)
on the surface of the particles, forming a surface layer comprising
a lithium metal composite oxide having a spinel crystal structure
represented by space group Fd-3m.
13. The process for producing a positive electrode active material
for a secondary battery according to claim 12, wherein the particle
forming step (A) comprises forming a particle comprising a lithium
metal composite oxide having a crystal structure represented by
space group P4.sub.332 and represented by formula (2):
Li.sub.a2(M2.sub.x2Mn.sub.2-x2-y2Y2.sub.y2)O.sub.4 (2) wherein,
0.4.ltoreq.x2.ltoreq.1.2, 0.ltoreq.y2.ltoreq.1, x2+y2<2,
0.ltoreq.a2.ltoreq.1.2; and M2 comprises at least one selected from
the group consisting of Co, Ni, Fe, Cr and Cu; Y2 is at least one
selected from the group consisting of Li, B, Na, Al, Mg, Ti, Si, K
and Ca; and the surface layer forming step (B) comprises forming on
a surface of the particle a surface layer comprising a lithium
metal composite oxide having a crystal structure represented by
space group Fd-3m and represented by formula (1):
Li.sub.a1(M1.sub.x1Mn.sub.2-x1-y1 Y1.sub.y1)O.sub.4 (1) wherein,
0.4.ltoreq.x1.ltoreq.1.2, 0.ltoreq.y1.ltoreq.1, x1+y1<2,
0.ltoreq.a1.ltoreq.1.2; M1 comprises at least one selected from the
group consisting of Co, Ni, Fe, Cr and Cu; Y1 is at least one
selected from the group consisting of Li, B, Na, Al, Mg, Ti, Si, K
and Ca.
14. The process for producing a positive electrode active material
for a secondary battery according to claim 13, comprising: in the
step (A), forming the particle comprising the lithium metal
composite oxide, and thereafter, in the step (B), to the particle
prepared in the step (A), adhering solution, dispersion or powder
comprising Li, Mn, an element represented by M1 in formula (1), and
if y1 is not 0 an element represented by Y1 in formula (1), and
subjecting drying and calcining.
15. The process for producing a positive electrode active material
for a secondary battery according to claim 13, wherein the surface
layer having a crystal structure of Fd-3m is formed in a thickness
in a range of 1 nm or more and 1 .mu.m or less.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
This application is a National Stage Entry of International
Application No. PCT/JP2014/079370, filed Nov. 5, 2014, which claims
priority from Japanese Patent Application No. 2013-250861, filed
Dec. 4, 2013. The entire contents of the above-referenced
applications are expressly incorporated herein by reference.
TECHNICAL FIELD
The present invention relates a secondary battery positive
electrode active material, and more particularly to a positive
electrode active material having a spinel crystal structure; and
further relates to a method for producing the same and a positive
electrode and a secondary battery using the positive electrode
active material.
BACKGROUND ART
Lithium secondary batteries (inclusive of lithium ion secondary
batteries) are widely utilized in portable electronic equipment,
personal computers, and the like. While miniaturization and weight
reduction are required for the lithium secondary batteries,
increasing the energy density is an important problem to be
solved.
There are several methods for increasing the energy density of a
lithium secondary battery, and among them, increasing the operating
voltage of a battery is effective. A lithium secondary battery
using lithium cobaltate or lithium manganate as a positive
electrode active material has an average operating voltage of 3.6
to 3.8 V (4 V class) versus a metal lithium reference. This is
because the operating voltage is defined by the oxidation-reduction
reaction of cobalt ions or manganese ions
(Co.sup.3+.revreaction.Co.sup.4+ or
Mn.sup.3+.revreaction.Mn.sup.4+).
On the other hand, a spinel compound in which a part of manganese
in lithium manganate is replaced by nickel or the like,
specifically LiNi.sub.0.5Mn.sub.1.5O.sub.4 or the like, shows a
potential plateau in a region of 4.5 V or more. Therefore, by using
the spinel compound of this type as a positive electrode active
material, 5 V class operating voltage can be achieved. In a
positive electrode using the spinel compound, manganese is present
in the tetravalent state, and the operating voltage of the battery
is defined by the oxidation-reduction of
Ni.sup.2+.revreaction.Ni.sup.4+ instead of the oxidation-reduction
of Mn.sup.3+.revreaction.Mn.sup.4+.
LiNi.sub.0.5Mn.sub.1.5O.sub.4 has a capacity of 130 mAh/g or more
and an average operating voltage of 4.6 V or more versus metal
lithium, and has smaller lithium absorbing capacity than
LiCoO.sub.2 but has higher energy density than LiCoO.sub.2. For
such a reason, LiNi.sub.0.5Mn.sub.1.5O.sub.4 is promising as a
positive electrode material.
On the other hand, improvement of the life characteristics is a
problem that has always been required in lithium batteries. Various
causes are said to be the reason of deterioration of battery life.
For example, the decomposition reaction of the electrolytic
solution at a contact portion of a positive electrode active
material with the electrolyte solution has been pointed out.
In order to suppress the decomposition on the positive electrode
active material, there are several techniques for treating the
surface of the positive electrode active material. For example,
there is a proposal to cover the surface of an active material with
a metal oxide, as disclosed in Patent Document 1 and Patent
Document 2.
As the reports relating to crystallinity of 5V class positive
electrodes, Non-Patent Document 1 and Non-Patent Document 2 are
known. Non-Patent Document 1 has shown characteristics such as
differences in interfacial resistance depending on the difference
in crystal structure due to presence or absence of Ni ordering.
Non-Patent Document 2 has shown that the crystal structure of
P4.sub.332 is obtained even when Mn was replaced with Ti in
LiNi.sub.0.5Mn.sub.1.5O.sub.4. Thus, it has been reported that the
reactivity with an electrolyte solution at the interface is
different depending on the control of the crystal structure, or
that it is possible to control the crystallinity of an active
material by conditions.
CITATION LIST
Patent Literature
Patent Document 1: Japanese Patent No. 4770113 Patent Document 2:
Japanese Patent No. 4683527
Non-Patent Literature
Non-Patent Document 1: Journal of The electrochemical Society, 160
(9) A1482-A1488 (2013) Non-Patent Document 2: Journal of The
electrochemical Society, 151 (11) A1911-A1918 (2013)
SUMMARY OF INVENTION
Technical Problem
It is an object of the present invention to provide a positive
electrode active material having a crystal structure of spinel type
with improved life characteristics as a positive electrode active
material for a secondary battery, and a secondary battery.
Solution to Problem
The present invention relates to a positive electrode active
material for a secondary battery, comprising:
(a) a surface layer comprising a lithium metal composite oxide
having a spinel crystal structure represented by space group Fd-3m,
and
(b) an internal portion comprising a lithium metal composite oxide
having a spinel crystal structure represented by space group
P4.sub.332.
Advantageous Effect of Invention
The constitution of the present invention provides a secondary
battery having an improved life characteristic. The present
invention is effective in improving the life characteristic of the
positive electrode active material having a spinel crystal
structure, in particular improving the life characteristic of those
containing a positive electrode active material operable at high
potential of 4.5V or more vs. lithium, without sacrificing its
energy density.
BRIEF DESCRIPTION OF DRAWING
FIG. 1 is one example of a diagram showing the cross-sectional
structure of a secondary battery according to the present
embodiment.
FIG. 2 is a diagram showing an electron beam diffraction pattern of
the internal portion of a particle of positive electrode active
material 1.
FIG. 3 is a diffraction pattern at[1-10]-incident for
Li.sub.2ZnMn.sub.3O.sub.8 type structure corresponding to the
crystal structure of the P4.sub.332.
FIG. 4 is a diffraction pattern at[1-10]-incident for spinel
(MgAl.sub.2O.sub.4) type structure.
DESCRIPTION OF EMBODIMENT
Particularly, a battery of high-voltage operation, for example, a
battery using a positive electrode active material of a high
potential, such as LiNi.sub.0.5Mn.sub.1.5O.sub.4, is operated at
further higher voltage than batteries using a positive electrode
active material such as LiCoO.sub.2 or LiMn.sub.2O.sub.4.
Therefore, the decomposition reaction of an electrolyte solution
tends to proceed at the contact portion of the positive electrode
with the electrolyte solution. Gas is generated by this
decomposition reaction. The generation of gas is a practical
problem because it increases the internal pressure of the cell or
causes a swelling of the laminate cell. Therefore, the development
of a positive electrode material effective to suppress the
decomposition of the electrolyte is expected. Since the
decomposition of the electrolytic solution occurs mainly at the
interface of the positive electrode active material and the
electrolytic solution, it is very important to control the surface
state of the positive electrode active material. As a method to
improve the properties of LiNi.sub.0.5Mn.sub.1.5O.sub.4, it is also
important that the positive electrode has a high crystallinity. The
high crystallinity enables smooth insertion and desorption of
Li.
The present inventors found, after investigating materials
excellent in life characteristics, that the effect of improving the
life characteristics is obtained by controlling the inside crystal
structure and the surface crystal state of the particle of the
positive electrode active material.
The secondary battery according to the present embodiment comprises
a positive electrode active material particle, in which the
internal portion of the positive electrode active material particle
has a crystal structure belonging to space group P4.sub.332, the
surface of the active material particle has a crystal structure
belonging to space group Fd-3m. In particular, it was found that a
high effect of improving lifetime characteristics is obtained
without compromising the energy density when a positive electrode
material operable at high potential of 4.5V or more vs. lithium is
used.
Preferred embodiments of the present invention will be
described.
(Positive Electrode Active Material)
In this embodiment, the positive electrode active material is a
lithium metal composite oxide having a surface layer and a particle
inner portion, wherein the surface layer and the particle inner
portion have a spinel crystal structure represented by space group
Fd-3m structure and a spinel crystal structure represented by space
group P4.sub.332, respectively.
In a more preferred embodiment, the surface layer and the particle
inner portion have chemical compositions represented by formula (1)
and (2), respectively, namely,
Li.sub.a1(M1.sub.x1Mn.sub.2-x1-y1Y1.sub.y1)O.sub.4 (1) wherein,
0.4.ltoreq.x1.ltoreq.1.2, 0.ltoreq.y1.ltoreq.1, x1+y1<2,
0.ltoreq.a1.ltoreq.1.2; M1 comprises at least one selected from the
group consisting of Co, Ni, Fe, Cr and Cu; Y1 is at least one
selected from the group consisting of Li, B, Na, Al, Mg, Ti, Si, K
and Ca; and Li.sub.a2(M2.sub.x2Mn.sub.2-x2-y2Y2.sub.y2)O.sub.4 (2)
wherein, 0.4.ltoreq.x2.ltoreq.1.2, 0.ltoreq.y2.ltoreq.1,
x2+y2<2, 0.ltoreq.a2.ltoreq.1.2; and M2 comprises at least one
selected from the group consisting of Co, Ni, Fe, Cr and Cu; Y2 is
at least one selected from the group consisting of Li, B, Na, Al,
Mg, Ti, Si, K and Ca. With such a positive electrode material,
characteristics such as a high capacity and high operational
potential can be obtained.
The internal portion of the positive electrode active material
particle is preferably a crystal structure of P4.sub.332. The
crystal structure of the P4.sub.332 is similar to Fd-3m. However,
taking LiNi.sub.0.5Mn.sub.1.5O.sub.4 as an example, in the lattice
point arrangement of Ni and Mn located in 16d sites of Fd-3m, if
ordering of Ni is advanced (regularly arranged), a structure of
P4.sub.332 is generated. It has been reported that depending on
manufacturing conditions such as the calcination conditions,
P4.sub.332 is generated or crystallinity stays in Fd-3m. If a
highly crystalline material in which the ordering of Ni is advanced
like this material is used, the conductivity of Li ion is improved.
This is considered to be due to the increase of diffusion rate of
Li during the charging and discharging of the battery.
On the other hand, in the surface of particles of the positive
electrode active material, the crystal structure of Fd-3m is
preferable. It is assumed that, with Fd-3m structure rather than
P4.sub.332 structure, the insertion and desorption of Li at the
interface of the electrolyte solution and positive electrode
surface takes place easily, and as a result, there is an effect of
suppressing side reactions of the electrolytic solution at the
interface of the electrolyte and the positive electrode.
M1 in formula (1) and M2 in formula (2) is preferably at least one
selected from Ni, Cu, Cr, Fe and Co. The use of such elements
enables the operation at high potential in the insertion and
desorption of Li.
M1 and M2, each preferably comprise at least Ni, and it is also
preferable M1 and M2 are Ni. By using Ni, an active material having
high operation potential with high capacity can be obtained.
In formula (2), 0.45<x2<0.55 is preferred. With such a range,
the structure of P4.sub.332 can be easily obtained. On the other
hand, even in the equation (1), 0.45<x1<0.55 is very
preferable. However, since there is no need of ordering of the M1
element in the structure of Fd-3m, the range of roughly
0.45<x1<0.8 is also a preferred range. With such a range, the
surface portion also becomes an active material operable at a high
potential with high capacity.
In formulae (1) and (2), Y1 and Y2 each are at least one element
selected from Li, B, Na, Al, Mg, Ti, Si, K and Ca, and y1
preferably satisfies 0.ltoreq.y1.ltoreq.0.3 and y2 preferably
satisfies 0.ltoreq.y2.ltoreq.0.3. These elements replace a part of
Mn sites. By the substitution by such elements, elution of the
constituent elements of the positive electrode active material such
as Mn can be suppressed. More preferably, Y1 and Y2 are selected
from Li, B, Mg, Al, Si and Ti. This is because these elements are
effective in improving performances through the reduction of the
elution. Ti is more preferable because it easily replaces Mn, and
the decrease in capacity is small even if the amount of
substitution is large and it is highly effective in reducing the
dissolution of the constituent elements.
The same effect can be obtained even if a part of oxygen of the
compound represented by formula (1) and formula (2) is being
substituted with small amounts of fluorine or chlorine. Further,
the same effect can be obtained even if the compound is in a such
state that a small amount of oxygen is deficient.
The positive electrode active materials present in the inside and
the surface may have the same composition or different composition.
When they are different, the composition of formula (1)
constituting the surface layer preferably has a similar composition
to formula (2) constituting the inner part of the active material.
For example, it is preferable that the elements M1 and M2 are the
same. Additionally thereto or independently thereof, the preference
is given to the cases that elements Y1 and Y2 are the same
(including the case where both elements are not contained), or Y1
is contained (y1.noteq.0) and Y2 is not contained (y2=0). Further,
if Y1 is contained, preferably y2/y1.ltoreq.2 is satisfied
(including the case of y2=0).
Because the component of the surface has such a composition that
represented by general formula (1), the insertion and desorption of
Li takes place in the surface portion, too. If the surface covering
component is an electrochemically inactive compound, the capacity
is reduced correspondingly. However, because having such a
composition that represented by general formula (1), it is possible
to suppress the reduction in capacity. Further, since the surface
and the internal portion of the crystal structure are similar, the
diffusion of Li is facilitated even at the interface between the
surface component and the internal component. Owing to these
effects, it is possible to suppress a reduction in charge and
discharge characteristics.
Further, when considering the reduction of the elution of the
constituent elements from the active material, the value of y1 in
general formula (1) of the compound in the surface is preferably
larger than y2 in general formula (2) of the compound in the
internal portion of the particle. By satisfying this, it is
possible to reduce the elution of elements from the active
material.
The thickness of the surface layer is preferably 1 nm or more and 1
.mu.m or less. This is because if it is 1 nm or less, the effect of
the surface layer is reduced, and if it is 1 .mu.m or more, the
effect due to the crystal structure of the internal portion is
lowered. More preferably, it is 2 nm or more, still more preferably
it is 5 nm or more.
The specific surface area of these positive active material is, for
example, 0.01 to 5 m.sup.2/g, preferably 0.02 to 4 m.sup.2/g, more
preferably 0.05 to 3 m.sup.2/g, further more preferably 0.1 to 2
m.sup.2/g. If the specific surface area is within such a range, the
contact area with the electrolytic solution can be adjusted in an
appropriate range. Namely, by setting the specific surface area to
0.01 m.sup.2/g or more, smooth insertion and desorption of lithium
ions proceeds easily, leading to further reduction in resistance.
Further, by setting the specific surface area to 5 m.sup.2/g or
less, the promotion of the decomposition of the electrolyte
solution and the elution of the constituent elements from the
active material can be prevented.
The median particle diameter of the positive electrode active
material is preferably 0.01 to 50 .mu.m, more preferably 0.02 to 40
.mu.m. By setting the particle size to 0.02 .mu.m or more, the
elution of the constituent elements from the positive electrode
material and the deterioration due to contact with the electrolyte
can be further suppressed. Further, by setting the particle
diameter to 50 .mu.m or less, smooth insertion and desorption of
lithium ions proceeds easily, leading to further reduction in
resistance. Particle size may be measured by laser
diffraction-diffusion particle size distribution measuring
apparatus.
In order to prepare a positive electrode active material of the
present embodiment, for example, preference is given to a
production method having a two-stage process including the steps
(A) and (B), as follows. That is, in step (A), particles of the
lithium metal composite oxide corresponding to the internal portion
of the positive electrode active material are formed, wherein the
lithium metal composite oxide is represented by formula (2) and has
a crystal structure represented by space group P4.sub.332. The
particle size and shape thereof are adjusted in consideration of
the size and shape of the final particles.
Method of forming crystals of P4.sub.332 is not particularly
limited, but it can be carried out, for example, by mixing a
predetermined amount of compounds containing metal elements
constituting the inner portion, and if necessary calcining the
mixture at elevated temperature (e.g. 800.degree. C. to
1100.degree. C.), then annealing it in a range of about 400.degree.
C. to 700.degree. C., preferably 500.degree. C. to 700.degree. C.
The annealing time can be appropriately set, but it may be within a
range of, for example, 5 hours to 100 hours. It is preferred that
the calcining and annealing are carried out in oxidation
atmosphere, particularly in an oxygen-rich condition (e.g. in
oxygen gas). The examples of the compounds containing metal
element(s) include sulfates, nitrates, chlorides, carbonates,
oxides, hydroxides, complexes, organic metal compounds and the
like.
Then, on the surface of the particles thus formed, lithium metal
composite oxide represented by formula (1) and having a crystal
structure represented by space group Fd-3m is formed. In this case,
it is preferable to form the surface layer so as to cover the
entire surface of the particles. Specific methods that can be
utilized include a sol-gel method or a neutralization method using
precursor solution(s), reaction methods such as a hydrothermal
method, a spray coating of colloidal dispersions, a hetero
aggregation method using a difference in surface charges in liquid,
wet methods such as a coating (or dipping) of precursor solution(s)
or dispersion(s), a solid phase method by powder mixing,
vapor-phase methods such as CVD and the like. The precursor means
compounds containing metal element(s) constituting the surface
layer, such as sulfates, nitrates, chlorides, carbonates, oxides,
hydroxides, complexes, and organometallic compounds.
After depositing compounds containing metal elements on the surface
of the particles, while not particularly limited, the particles are
calcined at an elevated temperature as required (e.g. 800.degree.
C. to 1100.degree. C.), and then annealed at 200 to 800.degree. C.,
for example at 700.degree. C. to 800.degree. C. The annealing time
can be set appropriately, but it is, for example, 1 hour to 30
hours. The calcination and the annealing are preferable carried out
in an oxidizing atmosphere. In other words, the condition of the
calcination and the annealing are selected so that the surface
layer becomes spinel type crystals, but ordering of M1 element
leading to P4.sub.332 structure does not take place and therefore
the surface layer stays in Fd-3m structure.
In this way, particles of a positive electrode active material
containing a lithium metal composite oxide is obtained, wherein the
internal portion comprises a lithium metal composite oxide having a
spinel crystal structure represented by space group P4.sub.332 and
a surface layer comprises a lithium metal composite oxide having a
spinel crystal structure represented by space group Fd-3m. Not
limited to the above methods, it is also possible to form a
structure having predetermined internal portion and surface layer,
for example by using a vapor phase method such as CVD. Forms of the
active material in this case are not limited to particles and may
be a layered structure.
(Positive Electrode)
Conductive imparting agents for positive electrodes that may be
used include acetylene black, carbon black, graphite, or other
carbon materials such as fibrous carbons, and in addition, metal
materials such as Al, powder of conductive oxides or the like.
Positive electrode binders are not particularly limited, but the
examples thereof include polyvinylidene fluoride (PVdF), vinylidene
fluoride-hexafluoropropylene copolymer, vinylidene
fluoride-tetrafluoroethylene copolymer, styrene-butadiene copolymer
rubber, polytetrafluoroethylene, polypropylene, polyethylene,
polyimides, polyamide-imides and the like. Among them,
polyvinylidene fluoride is preferred from the viewpoint of
versatility and low cost.
As the positive electrode active materials, mainly those
represented by formula (2) for the internal portion of the
particles and those represented by formula (1) for the surface
layer of the particles are used (preferably 70 wt % or more, more
preferably 90 wt % or more based on the total of the positive
electrode active material), but other active materials may be mixed
with them. As other positive electrode active materials that may be
used, the examples thereof include positive electrode active
materials having a spinel structure represented by LiM.sub.2O.sub.4
(M includes at least one or more metal elements such as Mn) other
than those represented by formula (1) or formula (2); positive
electrode active materials having an olivine-type crystal structure
represented by LiMPO.sub.4 (M includes at least one or more metal
elements such as Fe and Mn); positive electrode active materials
having a layered structure represented by LiMO.sub.2 (M includes at
least one or more metal elements such as Ni, Co, Mn and Fe);
NASICON type; lithium-transition metal-silicon composite oxides and
the like. The positive electrode active materials may be used alone
or in mixture of two or more kinds.
The addition amount of the conductive imparting agent is preferably
from 0.5 to 10% by weight (based on the total amount of the
positive electrode active material, the conductive imparting agent
and the binder), and the addition amount of the binder is also 1 to
10% (based on the total amount of the positive electrode active
material, the conductive imparting agent and the binder). This is
because, if the proportion of the conductive imparting agent and
the binder is small, problems such as poor electronic conductivity
or separation of the electrode tend to occur. And if the proportion
of the conductive imparting agent and a binder is large, the
capacity per cell mass is reduced. The proportion of the positive
electrode active material is preferably 70 to 99 wt % (based on the
total amount of the positive electrode active material, the
conductive imparting agent and the binder), and more preferably 88
to 98% (based on the total amount of the positive electrode active
material, the conductive imparting agent and the binder). If the
proportion of the positive electrode active material is too small,
it is disadvantageous in terms of energy density of the battery. If
the proportion of the active material is too large, it is
disadvantageous in that problems such as poor electronic
conductivity or separation of the electrode tend to occur because
the proportion of the conductive imparting agent and binder per
mass becomes less.
As the positive electrode current collector, thin metal films
composed mainly of Al or the like is preferred. The examples of
shapes thereof include a foil, flat plate, mesh and the like.
The positive electrode may be produced by, on a positive electrode
current collector, forming a positive electrode active material
layer containing a positive electrode active material and a
positive electrode binder. As a method of forming the positive
electrode active material layer, the examples include a doctor
blade method, a die coater method, CVD method, and a sputtering
method. After forming the positive electrode active material layer
in advance, a positive electrode current collector may be provided
by forming a thin film of aluminum, nickel or alloys of these by
method such as vapor deposition, sputtering and the like.
(Electrolyte Solution)
As a solvent of the electrolytic solution in the present invention,
carbonate-based compounds, carboxylic acid ester compounds, ether
compounds, phosphoric acid ester compounds, sulfone compounds and
the like may be used. Each solvent may be an open-chain structure
or a cyclic structure. Further, these compounds may be partially
substituted with an element such as fluorine chlorine. Further, a
part of elements may be substituted with a cyano group, an imide
group or the like.
The carbonate-based compound are roughly divided into open-chain
carbonates and cyclic carbonates.
The cyclic carbonate is not particularly limited. Examples thereof
may include ethylene carbonate (EC), propylene carbonate (PC),
butylene carbonate (BC), or vinylene carbonate (VC). In addition,
the cyclic carbonate includes a fluorinated cyclic carbonate.
Examples of the fluorinated cyclic carbonate include compounds
obtainable by replacing some or all hydrogen atoms of ethylene
carbonate (EC), propylene carbonate (PC), butylene carbonate (BC),
vinylene carbonate (VC), or the like by fluorine atoms. More
specifically, as the fluorinated cyclic carbonate,
4-fluoro-1,3-dioxolan-2-one, (cis or
trans)4,5-difluoro-1,3-dioxolan-2-one,
4,4-difluoro-1,3-dioxolan-2-one,
4-fluoro-5-methyl-1,3-dioxolan-2-one, or the like may be used.
Among those listed above, from the viewpoint of voltage resistance
and conductivity, ethylene carbonate, propylene carbonate, or
compounds obtainable by fluorinating parts of these, or the like is
preferred, and ethylene carbonate is more preferred. One cyclic
carbonate may be used alone, or two or more cyclic carbonates may
be used in combination.
The open-chain carbonate is not particularly limited. Examples
thereof include dimethyl carbonate (DMC), ethyl methyl carbonate
(EMC), diethyl carbonate (DEC), and dipropyl carbonate (DPC). In
addition, the open-chain carbonate includes a fluorinated
open-chain carbonate. Examples of the fluorinated open-chain
carbonate include compounds obtainable by replacing some or all
hydrogen atoms of ethyl methyl carbonate (EMC), dimethyl carbonate
(DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), and the
like by fluorine atoms. More specifically, examples of the
fluorinated open-chain carbonate include bis(fluoroethyl)
carbonate, 3-fluoropropyl methyl carbonate, and
3,3,3-trifluoropropyl methyl carbonate. Among these, dimethyl
carbonate and diethyl carbonate are preferred from the viewpoint of
voltage resistance and conductivity. One open-chain carbonate may
be used alone, or two or more open-chain carbonates may be used in
combination.
The carboxylate is not particularly limited. Examples thereof
include ethyl acetate, methyl propionate, ethyl formate, ethyl
propionate, methyl butyrate, ethyl butyrate, methyl acetate, and
methyl formate. In addition, the carboxylate includes a fluorinated
carboxylate. Examples of the fluorinated carboxylate include
compounds obtainable by replacing some or all hydrogen atoms of
ethyl acetate, methyl propionate, ethyl formate, ethyl propionate,
methyl butyrate, ethyl butyrate, methyl acetate, or methyl formate
by fluorine atoms. Specific examples thereof include ethyl
pentafluoropropionate, ethyl 3,3,3-trifluoropropionate, methyl
2,2,3,3-tetrafluoropropionate, 2,2-difluoroethyl acetate, methyl
heptafluoroisobutyrate, methyl 2,3,3,3-tetrafluoropropionate,
methyl pentafluoropropionate, methyl
2-(trifluoromethyl)-3,3,3-trifluoropropionate, ethyl
heptafluorobutyrate, methyl 3,3,3-trifluoropropionate,
2,2,2-trifluoroethyl acetate, isopropyl trifluoroacetate,
tert-butyl trifluoroacetate, ethyl 4,4,4-trifluorobutyrate, methyl
4,4,4-trifluorobutyrate, butyl 2,2-difluoroacetate, ethyl
difluoroacetate, n-butyl trifluoroacetate,
2,2,3,3-tetrafluoropropyl acetate, ethyl
3-(trifluoromethyl)butyrate, methyl
tetrafluoro-2-(methoxy)propionate, 3,3,3trifluoropropyl
3,3,3-trifluoropropionate, methyl difluoroacetate,
2,2,3,3-tetrafluoropropyl trifluoroacetate, 1H,1H-heptafluorobutyl
acetate, methyl heptafluorobutyrate, and ethyl trifluoroacetate.
Among these, ethyl propionate, methyl acetate, methyl
2,2,3,3-tetrafluoropropionate, 2,2,3,3-tetrafluoropropyl
trifluoroacetate, and the like are preferred from the viewpoint of
voltage resistance, the boiling point, and the like.
The cyclic carboxylic acid ester is not particularly limited, but
the preferred examples thereof include .gamma.-lactones such as
.gamma.-butyrolactone, .alpha.-methyl-.gamma.-butyrolactone and
3-methyl-.gamma.-butyrolactone, and .beta.-propiolactone,
.delta.-valerolactone and the like. Fluorinated compounds and the
like of these may also be used.
The open-chain ether is not particularly limited. Examples thereof
include 1,2-ethoxyethane (DEE) or ethoxymethoxyethane (EME). In
addition, a fluorinated open-chain ether obtainable by replacing
part of the hydrogen of a open-chain ether by fluorine has high
oxidation resistance and is preferred for a positive electrode
operating at high potential.
Examples of the fluorinated open-chain ether include
2,2,3,3,3-pentafluoropropyl 1,1,2,2-tetrafluoroethyl ether,
1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether,
1H,1H,2'H,3H-decafluorodipropyl ether,
1,1,1,2,3,3-hexafluoropropyl-2,2-difluoroethyl ether, isopropyl
1,1,2,2-tetrafluoroethyl ether, propyl 1,1,2,2-tetrafluoroethyl
ether, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether,
1H,1H,5H-perfluoropentyl-1,1,2,2-tetrafluoroethyl ether,
1H,1H,2'H-perfluorodipropyl ether,
1H-perfluorobutyl-1H-perfluoroethyl ether, methyl perfluoropentyl
ether, methyl perfluorohexyl ether, methyl
1,1,3,3,3-pentafluoro-2-(trifluoromethyppropyl ether,
1,1,2,3,3,3-hexafluoropropyl 2,2,2-trifluoroethyl ether, ethyl
nonafluorobutyl ether, ethyl 1,1,2,3,3,3-hexafluoropropyl ether,
1H,1H,5H-octafluoropentyl 1,1,2,2-tetrafluoroethyl ether, 1H, 1H,
2'H-perfluorodipropyl ether, heptafluoropropyl
1,2,2,2-tetrafluoroethyl ether,
1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether,
2,2,3,3,3-pentafluoropropyl-1,1,2,2-tetrafluoroethyl ether, ethyl
nonafluorobutyl ether, and methyl nonafluorobutyl ether,
1,1-difluoroethyl 2,2,3,3-tetrafluoropropyl ether, bis
(2,2,3,3-tetrafluoro propyl) ether, 1,1-difluoroethyl
2,2,3,3,3-pentafluoropropyl ether, 1,1-difluoroethyl
1H,1H-heptafluorobutyl ether, 2,2,3,4,4,4-hexafluorobutyl
difluoromethyl ether, bis (2,2,3,3,3-pentafluoro-propyl) ether,
nonafluorobutyl methyl ether, bis(1H,1H-heptafluorobutyl) ether,
1,1,2,3,3,3-hexafluoropropyl-1H,1H-heptafluorobutyl ether,
1H,1H-heptafluorobutyl trifluoromethyl ether, 2,2-difluoroethyl
1,1,2,2-tetrafluoroethyl ether, bis(trifluoroethyl) ether,
bis(2,2-difluoroethyl) ether, bis(1,1,2-trifluoroethyl) ether,
1,1,2-trifluoroethyl 2,2,2-trifluoroethyl ether.
Among these, from the viewpoint of voltage resistance, the boiling
point, and the like, 1,1,2,2-tetrafluoroethyl
2,2,3,3-tetrafluoropropyl ether, 2,2,3,4,4,4-hexafluorobutyl
difluoromethyl ether, 1,1-difluoroethyl 2,2,3,3-tetrafluoropropyl
ether, 1,1,1,2,3,3-hexafluoropropyl 2,2-difluoro ethyl ether,
1,1-difluoroethyl 1H,1H-heptafluorobutyl ether,
1H,1H,2'H,3H-decafluorodipropyl ether,
bis(2,2,3,3,3-pentafluoropropyl) ether, 1H,1H,5H-perfluoropentyl
1,1,2,2-fluoroethyl ether, bis(1H,1H-heptafluorobutyl) ether,
1H,1H,2'H-perfluorodipropyl ether, 1,1,2,3,3,3-hexafluoropropyl
1H,1H-heptafluorobutyl ether, 1H-perfluorobutyl 1H-perfluoroethyl
ether is preferred
The cyclic ether compounds include cyclic ethers such as
tetrahydrofuran, 2-methyltetrahydrofuran,
2,2-dimethyltetrahydrofuran and the like. Materials in which a part
of these compounds are substituted with fluorine may also be
used.
Examples of the phosphate include trimethyl phosphate, triethyl
phosphate, and tributyl phosphate. Examples of the
fluorine-containing phosphate include 2,2,2-trifluoroethyl dimethyl
phosphate, bis(trifluoroethyl) methyl phosphate, his trifluoroethyl
ethyl phosphate, tris(trifluoromethyl) phosphate, pentafluoropropyl
dimethyl phosphate, heptafluorobutyl dimethyl phosphate,
trifluoroethyl methyl ethyl phosphate, pentafluoropropyl methyl
ethyl phosphate, heptafluorobutyl methyl ethyl phosphate,
trifluoroethyl methyl propyl phosphate, pentafluoropropyl methyl
propyl phosphate, heptafluorobutyl methyl propyl phosphate,
trifluoroethyl methyl butyl phosphate, pentafluoropropyl methyl
butyl phosphate, heptafluorobutyl methyl butyl phosphate,
trifluoroethyl diethyl phosphate, pentafluoropropyl diethyl
phosphate, heptafluorobutyl diethyl phosphate, trifluoroethyl ethyl
propyl phosphate, pentafluoropropyl ethyl propyl phosphate,
heptafluorobutyl ethyl propyl phosphate, trifluoroethyl ethyl butyl
phosphate, pentafluoropropyl ethyl butyl phosphate,
heptafluorobutyl ethyl butyl phosphate, trifluoroethyl dipropyl
phosphate, pentafluoropropyl dipropyl phosphate, heptafluorobutyl
dipropyl phosphate, trifluoroethyl propyl butyl phosphate,
pentafluoropropyl propyl butyl phosphate, heptafluorobutyl propyl
butyl phosphate, trifluoroethyl dibutyl phosphate,
pentafluoropropyl dibutyl phosphate, heptafluorobutyl dibutyl
phosphate, tris(2,2,3,3-tetrafluoropropyl) phosphate,
tris(2,2,3,3,3-pentafluoropropyl) phosphate,
tris(2,2,2-trifluoroethyl) phosphate (hereinafter, also abbreviated
as PTTFE), tris(1H,1H-heptafluorobutyl) phosphate, and
tris(1H,1H,5H-octafluoropentyl) phosphate.
Examples of sulfone compounds that may be used include sulfolane
(tetramethylene sulfone), 3-methyl sulfolane and the like as cyclic
sulfone-based materials. Examples of open-chain sulfone-based
materials include dimethyl sulfone (for example, 3,4-dimethyl
sulfone, 2,5-dimethyl sulfone), ethyl methyl sulfone, diethyl
sulfone, butyl methyl sulfone, dibutyl sulfone, methyl isopropyl
sulfone, diisopropyl sulfone, methyl tert-butyl sulfone, butyl
ethyl sulfone, butyl propyl sulfone, butyl isopropyl sulfone,
di-tert-butyl sulfone, diisobutyl sulfone, ethyl isopropyl sulfone,
ethyl isobutyl sulfone, tert-butyl ethyl sulfone, propyl ethyl
sulfone, isobutyl isopropyl sulfone, butyl isobutyl sulfone,
isopropyl (1-methyl-propyl) sulfone, pentamethylene sulfone,
hexamethylene sulfone, ethylene sulfone, trimethylene sulfone, and
the like. These compounds may be used alone or in combination of
two or more of these.
Examples of the supporting salt in the electrolyte solution include
lithium salts, such as LiPF.sub.6, LiAsF.sub.6, LiAlCl.sub.4,
LiClO.sub.4, LiBF.sub.4, LiSbF.sub.6, LiCF.sub.3SO.sub.3,
LiC.sub.4F.sub.9SO.sub.3, LiC(CF.sub.3SO.sub.2).sub.2,
LiN(CF.sub.3SO.sub.2).sub.2, LiN(C.sub.2F.sub.5SO.sub.2).sub.2, and
LiB.sub.10Cl.sub.10. In addition, examples of other supporting
salts include lithium lower aliphatic carboxylates, chloroborane
lithium, lithium tetraphenylborate, LiBr, LiI, LiSCN, and LiCl. One
supporting salt may be used alone, or two or more supporting salts
may be used in combination.
As additives for electrolyte solution, carbonate-based compound
having an unsaturated bond such as vinylene carbonate (VC),
sulfonic acid ester compounds such as 1,3-propane sultone and
butane sultone may be used.
An ion-conducting polymer may be added to the nonaqueous
electrolytic solution. Examples of the ion-conducting polymer
include polyethers, such as polyethylene oxide and polypropylene
oxide, and polyolefins, such as polyethylene and polypropylene. In
addition, examples of the ion-conducting polymer include
polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl
fluoride, polyvinyl chloride, polyvinylidene chloride, polymethyl
methacrylate, polymethyl acrylate, polyvinyl alcohol,
polymethacrylonitrile, polyvinyl acetate, polyvinylpyrrolidone,
polycarbonates, polyethylene terephthalate, polyhexamethylene
adipamide, polycaprolactam, polyurethanes, polyethylenimine,
polybutadiene, polystyrene, or polyisoprene, or derivatives
thereof. One ion-conducting polymer may be used alone, or two or
more ion-conducting polymers may be used in combination. In
addition, polymers comprising various monomers forming the above
polymers may be used.
(Negative Electrode)
The negative electrode active material is not particularly limited.
Examples thereof include a carbon material capable of absorbing and
desorbing lithium ions (a), a metal capable of being alloyed with
lithium (b), or a metal oxide capable of absorbing and desorbing
lithium ions (c).
As the carbon material (a), graphite, amorphous carbon,
diamond-like carbon, carbon nanotubes, or composites thereof can be
used. Graphite having high crystallinity has high electrical
conductivity and has excellent adhesiveness to a negative electrode
current collector formed of a metal, such as copper, and excellent
voltage flatness. On the other hand, in amorphous carbon having low
crystallinity, the volume expansion is relatively small, and
therefore, the effect of relieving the volume expansion of the
entire negative electrode is large, and deterioration caused by
nonuniformity, such as grain boundaries and defects, does not occur
easily. The carbon material (a) can be used alone or in combination
with other materials.
As the metal (b), a metal mainly composed of Al, Si, Pb, Sn, Zn,
Cd, Sb, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Te, La, and the like, or
alloys comprising two or more of these, or alloys of these metals
or alloys with lithium, or the like can be used. Particularly, the
metal (b) preferably comprises silicon (Si). The metal (b) may be
used alone or in combination with other materials.
As the metal oxide (c), silicon oxide, aluminum oxide, tin oxide,
indium oxide, zinc oxide, lithium oxide, LiFe.sub.2O.sub.3,
WO.sub.2, MoO.sub.2, SiO, SiO.sub.2, CuO, SnO, SnO.sub.2,
Nb.sub.3O.sub.5, Li.sub.xTi.sub.2-xO.sub.4
(1.3.ltoreq.x.ltoreq.4/3), PbO.sub.2, Pb.sub.2O.sub.5, or
composites thereof can be used. Particularly, the metal oxide (c)
preferably comprises silicon oxide. This is because silicon oxide
is relatively stable and does not easily cause reactions with other
compounds. In addition, one or two or more elements selected from
among nitrogen, boron, and sulfur may also be added to the metal
oxide (c), for example, in an amount of 0.1 to 5% by mass. By doing
this, the electrical conductivity of the metal oxide (c) is
improved. The metal oxide (c) may be used alone or in combination
with other materials.
In addition, the negative electrode active materials may include,
for example, a metal sulfide capable of absorbing and desorbing
lithium ions. Examples of the metal sulfide include SnS and
FeS.sub.2. In addition, examples of the negative electrode active
material may include metal lithium, polyacene or polythiophene, or
lithium nitride, such as Li.sub.5(Li.sub.3N), Li.sub.7MnN.sub.4,
Li.sub.3FeN.sub.2, Li.sub.2.5Co.sub.0.5N, or Li.sub.3CoN.
The above negative electrode active materials may be used alone or
in a mixture of two or more of these.
As these negative electrode active materials, those in a form of
particles may be used, or those formed into a film by vapor-phase
deposition method or the like on a current collector may be used.
In terms of industrial applications, those in a form of particles
are preferable.
The specific surface area of particles of these negative electrode
active materials is, for example, 0.01 to 100 m.sup.2/g, preferably
0.02 to 50 m.sup.2/g, more preferably 0.05 to 30 m.sup.2/g and even
more preferably 0.1 to 20 m.sup.2/g. If the specific surface area
is within such a range, the contact area with the electrolytic
solution can be adjusted in an appropriate range. Namely, by
setting the specific surface area to 0.01 m.sup.2/g or more, smooth
insertion and desorption of lithium ions proceeds easily, leading
to further reduction in resistance. Further, by setting the
specific surface area to 20 m.sup.2/g or less, the promotion of the
decomposition of the electrolyte solution and the elution of the
constituent elements from the active material can be prevented.
The negative electrode binder is not particularly limited. Examples
thereof include polyvinylidene fluoride (PVdF), vinylidene
fluoride-hexafluoropropylene copolymers, vinylidene
fluoride-tetrafluoroethylene copolymers, styrene-butadiene
copolymerized rubbers, polytetrafluoroethylene, polypropylene,
polyethylene, polyimides, and polyamideimides.
The content of the negative electrode binder is preferably in the
range of 0.1 to 30% by mass, more preferably 0.5 to 25% by mass,
based on the total amount of the negative electrode active material
and the negative electrode binder. By setting the content to 0.5%
by mass or more, the adhesiveness between the active materials or
between the active material and the current collector is improved,
and the cycle characteristics are good. In addition, by setting the
content to 30% by mass or less, the active material ratio is
improved, and the negative electrode capacity can be improved.
The negative electrode current collector is not particularly
limited, and aluminum, nickel, copper, silver, iron, chromium, and
alloys thereof are preferred because of electrochemical stability.
Examples of its shape include foil, a flat plate shape, and a mesh
shape.
The negative electrode can be made by forming a negative electrode
active material layer comprising a negative electrode active
material and a negative electrode binder on a negative electrode
current collector. Examples of the method for forming the negative
electrode active material layer include a doctor blade method, a
die coater method, a CVD method, and a sputtering method. It is
possible to previously form a negative electrode active material
layer and then form a thin film of aluminum, nickel, or an alloy
thereof by a method such as vapor deposition or sputtering to
provide a negative electrode current collector.
(Separator)
The secondary battery may consist of a combination of a positive
electrode, a negative electrode, a separator, and a nonaqueous
electrolyte as its configuration. Examples of the separator include
woven fabrics, nonwoven fabrics, porous polymer films of
polyolefins, such as polyethylene and polypropylene, polyimides and
porous polyvinylidene fluoride films, and the like, or
ion-conducting polymer electrolyte films. These may be used alone
or in combination.
(Shape of Battery)
Examples of the shape of the battery include a cylindrical shape, a
rectangular shape, a coin type, a button type, and a laminate type.
Examples of the package of the battery include stainless, iron,
aluminum, titanium, or alloys thereof, or plated articles thereof.
As the plating, for example, nickel plating may be used.
Examples of the laminate resin film used in a laminate type include
aluminum, aluminum alloy, and titanium foil. Examples of the
material of the thermally bondable portion of the metal laminate
resin film include thermoplastic polymer materials, such as
polyethylene, polypropylene, and polyethylene terephthalate. In
addition, each of the numbers of the metal laminate resin films and
the metal foil layers is not limited to one and may be two or
more.
FIG. 1 shows a structure of a secondary battery according to the
present embodiment. The lithium secondary battery comprises a
positive electrode active material layer 1 containing a positive
electrode active material on a positive electrode current collector
3 formed of a metal, such as aluminum foil, and a negative
electrode active material layer 2 containing a negative electrode
active material on a negative electrode current collector 4 formed
of a metal, such as copper foil. The positive electrode active
material layer 1 and the negative electrode active material layer 2
are disposed opposed to each other via an electrolytic solution and
a separator 5 formed of a nonwoven fabric, a polypropylene
microporous film, or the like comprising the electrolytic solution.
In FIG. 1, reference numerals 6 and 7 denote a package, reference
numeral 8 denotes a negative electrode tab, and reference numeral 9
denotes a positive electrode tab.
EXAMPLES
(Preparation Conditions of Positive Electrode Active Materials)
(Preparation of Positive Electrode Active Material 1)
Raw materials MnO.sub.2 and NiO were ground and mixed so that the
molar ratio of the elements was Ni/Mn=0.5/1.5, and the mixture was
calcined at 950.degree. C. for 8 hours. The obtained NiMn composite
oxide and Li.sub.2CO.sub.3 were mixed so that the molar ratio of
Li, Mn and Ni was Li/Ni/Mn=1/0.5/1.5, and calcined in oxygen at
600.degree. C. for 48 hours.
(Preparation of Positive Electrode Active Material 2)
Raw materials Li.sub.2CO.sub.3, MnO.sub.2 and NiO were weighed so
that the metal composition ratio became a targeted value, and
ground and mixed. They were mixed so that the molar ratio of Li, Mn
and Ni became Li/Ni/Mn=1/0.5/1.5, and the mixture was calcined at
850.degree. C. for 8 hours.
(Preparation of Positive Electrode Active Material 3)
Acetate salts of lithium, manganese and nickel were mixed so that
Li/Ni/Mn=1/0.5/1.5 in molar ratio, and were dissolved in an aqueous
solution. This solution and positive electrode active material 1
were mixed so that the ratio of moles of Li+Ni+Mn in the solution
to moles of Li+Ni+Mn of the active material particles was adjusted
to be 1/9. After mixing, the mixture was dried in an oven at
80.degree. C., and calcined in air at 700.degree. C. for 8
hours.
The obtained positive electrode active material was evaluated with
XRD (X-ray diffraction) measurement. The diffraction patterns
similar to spinel structure considered
LiNi.sub.0.5Mn.sub.1.5O.sub.4 were observed in each case. These
particles were cut out to expose a cross section of the particles
by ion milling, and subjected to TEM observation. The surface and
the internal portion of the particle were evaluated with electron
diffraction.
FIG. 2 shows an electron beam diffraction pattern of an internal
portion of the particle of the positive electrode active material
1. FIG. 3 shows a diffraction pattern for[1-10] incidence of
Li.sub.2ZnMn.sub.3O.sub.8 type structure corresponding to the
crystal structure of P4.sub.332, and FIG. 4 shows a diffraction
pattern for[1-10] incidence of spinel (MgAl.sub.2O.sub.4) type
structure. Diffraction spots are seen at locations indicated by
arrows in FIG. 2. There are diffraction spots at locations
indicated by arrows in FIG. 3 similarly, while there is no such
spot at locations indicated by arrows in FIG. 4. From the results,
it was confirmed that the internal portion of the particles of the
positive electrode active material 1 has Li.sub.2ZnMn.sub.3O.sub.8
type structure (P4.sub.332 structure).
In a similar manner, the crystal structure was determined at an
internal portion of particles and a portion located inside by about
10 nm from the surface of particles as to whether it was P4.sub.332
or Fd-3m. For example, in the case of positive electrode active
material 2, since diffraction spots assigned to P4.sub.332 was not
observed, the crystal structure was determined as Fd-3m. The
results are shown in Table 1.
TABLE-US-00001 TABLE 1 crystal phase at internal crystal phase at
portion of particles surface of particles positive electrode
P4.sub.332 P4.sub.332 active material 1 positive electrode Fd-3m
Fd-3m active material 2 positive electrode P4.sub.332 Fd-3m active
material 3
As shown in Table 1, samples having different crystal structures
have been prepared. In the positive electrode active material 3,
there has been produced a positive electrode active material having
different crystal structures between at the internal portion and at
the surface of the particles.
The positive electrode active materials 1 to 3 thus obtained,
polyvinylidene fluoride (5 wt %) as a binder, carbon black (5 wt %)
as a conductive agent were mixed to prepare a positive electrode
mixture. The positive electrode mixture was dispersed in
N-methyl-2-pyrrolidone to prepare a positive electrode slurry. One
surface of a 20 .mu.m thick aluminum current collector was
uniformly coated with this positive electrode slurry. The thickness
of the coating film was adjusted so that the initial charge
capacity per unit area was 2.5 mAh/cm.sup.2. The coated current
collector was dried and then compression-shaped by a roll press to
make a positive electrode.
As a negative electrode active material, synthetic graphite was
used. The synthetic graphite was dispersed in N-methylpyrrolidone
in which PVDF is dissolved, to prepare a negative electrode slurry.
The mass ratio of the negative electrode active material to the
binder was 90/10. A 10 .mu.m thick Cu current collector was
uniformly coated with this negative electrode slurry. The thickness
of the coating film was adjusted so that the initial charge
capacity was 3.0 mAh/cm.sup.2. The coated current collector was
dried and then compression-shaped by a roll press to make a
negative electrode.
The positive electrode and the negative electrode cut into 3
cm.times.3 cm were disposed so as to be opposed to each other via a
separator. For the separator, a 25 .mu.m thick microporous
polypropylene film was used.
As the nonaqueous electrolytic solution, a solution obtained by
mixing ethylene carbonate (EC), tris(2,2,2-trifluoroethyl)
phosphate (FP1), and 1,1,2,2-tetrafluoroethyl
2,2,3,3-tetrafluoropropyl ether (FE1) at a volume ratio of
EC/FP1/FE1=20/30/50 was used. LiPF.sub.6 was dissolved in this
nonaqueous electrolytic solution at a concentration of 0.8 mol/l to
prepare a nonaqueous electrolytic solution.
The above positive electrode, negative electrode, separator, and
electrolytic solution were disposed in a laminate package, and the
laminate was sealed to make a lithium secondary battery. The
positive electrode and the negative electrode were brought into a
state in which tabs were connected and the positive electrode and
the negative electrode were electrically connected from the outside
of the laminate.
The battery using positive electrode active material 1 was denoted
as Comparative Example 1, the battery using positive electrode
active material 2 was denoted as Comparative Example 2, and the
battery using positive electrode active material 3 was denoted as
Example 1.
(Cycle Characteristics)
These batteries were charged at 20 mA, and after the voltage
reached the upper limit 4.75 V, the battery was charged at constant
voltage until the total charge time reached 2.5 hours. Then, the
batteries were discharged at 20 mA at constant current until a
lower limit voltage of 3 V was reached. This charge and discharge
was repeated 200 times. The cells were disposed in a thermostat
chamber at 45.degree. C., and charge and discharge were carried
out. The ratio of capacity at the 200th cycle to capacity at the
1st cycle was evaluated as capacity retention ratio after 200
cycles at 45.degree. C. The results are shown in Table 2.
TABLE-US-00002 TABLE 2 crystal positive structure at crystal
Capacity retention electrode internal structure ratio after active
portion of at surface of 200 cycles material particles particles at
45.degree. C. Comp. Ex. 1 positive P4.sub.332 P4.sub.332 62%
electrode active material 1 Comp. Ex. 2 positive Fd-3m Fd-3m 60%
electrode active material 2 Example 1 positive P4.sub.332 Fd-3m 71%
electrode active material 3
As shown in Table 2, the retention ratio is high in the case that
the surface of the active material has Fd-3m and internal portion
has P4.sub.332. The reason why this result was obtained is that
highly crystalline P4.sub.332 is preferable for the movement of Li
ions in the internal portion, whereas in the surface, Fd-3m is of
lower reactivity at the interface of the positive electrode with
the electrolytic solution and the insertion and desorption of Li
take place easily. In addition, insertion and desorption of Li are
possible also at the surface portion, and therefore the surface
portion is capable of performing charging and discharging. Owing to
this, there is no reduction in capacity attributed to the surface
coating component, and therefore it has an effect of maintaining a
high capacity compared with electrochemically inactive covering
component. Also, the structure of the internal portion of the
particles and the structure of the surface are similar, it is
considered that diffusion of Li ions has easily occurred.
Next, similar experiments were also carried out with other active
material compositions.
Comparative Example 3
Raw materials MnO.sub.2, NiO and TiO.sub.2 were ground and mixed so
that the molar ratio of the elements was Ni/Mn/Ti=0.5/1.4/0.1, and
the mixture was calcined at 950.degree. C. for 8 hours. The
obtained NiMnTi composite oxide and Li.sub.2CO.sub.3 were mixed so
that the molar ratio of Li, Mn, Ni and Ti was
Li/Ni/Mn/Ti=1/0.5/1.4/0.1, and calcined in oxygen at 600.degree. C.
for 48 hours. The crystal structure was evaluated in the same
manner as for positive electrode active material 1, and a battery
was manufactured in the same manner as in Comparative Example 1 to
prepare the sample of Comparative Example 3.
Comparative Example 4
Raw materials Li.sub.2CO.sub.3, MnO.sub.2, NiO and TiO.sub.2 were
ground and mixed so that the molar ratio of Li, Ni, Mn and Ti was
Li/Ni/Mn/Ti=1/0.5/1.4/0.1, and the mixture was calcined at
850.degree. C. for 8 hours. The crystal structure was evaluated in
the same manner as for positive electrode active material 1, and a
battery was manufactured in the same manner as in Comparative
Example 1 to prepare the sample of Comparative Example 4.
Example 2
Acetate salts of lithium, manganese, nickel and titanium were mixed
so that Li/Ni/Mn/Ti=1/0.5/1.4/0.1 in molar ratio, and were
dissolved in an aqueous solution. This solution and positive
electrode active material prepared in Comparative Example 3 were
mixed so that the ratio of moles of Li+Ni+Mn+Ti in the solution to
moles of Li+Ni+Mn+Ti of the active material particles was adjusted
to be 1/9. After mixing, the mixture was dried in an oven at
80.degree. C., and calcined in air at 700.degree. C. for 8 hours.
The crystal structure was evaluated in the same manner as for
positive electrode active material 1, and a battery was
manufactured in the same manner as in Comparative Example 1 to
prepare the sample of Example 2.
Example 3
Acetate salts of lithium, manganese, nickel and titanium were mixed
so that Li/Ni/Mn/Ti=1/0.5/1.2/0.3 in molar ratio, and were
dissolved in an aqueous solution. This solution and positive
electrode active material prepared in Comparative Example 3 were
mixed so that the ratio of moles of Li+Ni+Mn+Ti in the solution to
moles of Li+Ni+Mn+Ti of the active material particles was adjusted
to be 1/9. After mixing, the mixture was dried in an oven at
80.degree. C., and calcined in air at 700.degree. C. for 8 hours.
The crystal structure was evaluated in the same manner as for
positive electrode active material 1, and a battery was
manufactured in the same manner as in Comparative Example 1 to
prepare the sample of Example 3.
Example 4
Acetate salts of lithium, manganese, nickel and titanium were mixed
so that Li/Ni/Mn/Ti=1/0.5/1.2/0.3 in molar ratio, and were
dissolved in an aqueous solution. This solution and positive
electrode active material 1 were mixed so that the ratio of moles
of Li+Ni+Mn+Ti in the solution to moles of Li+Ni+Mn+Ti of the
active material particles was adjusted to be 1/9. After mixing, the
mixture was dried in an oven at 80.degree. C., and calcined in air
at 700.degree. C. for 8 hours. The crystal structure was evaluated
in the same manner as for positive electrode active material 1, and
a battery was manufactured in the same manner as in Comparative
Example 1 to prepare the sample of Example 3.
Comparative Example 5
Raw materials Li.sub.2CO.sub.3, MnO.sub.2, NiO and Al(OH).sub.3
were mixed so that the molar ratio of Li, Mn, Ni, Al was
Li/Ni/Mn/Al=1/0.5/1.48/0.02, and the mixture was calcined at
850.degree. C. for 8 hours. The crystal structure was evaluated in
the same manner as for positive electrode active material 1, and a
battery was manufactured in the same manner as in Comparative
Example 1 to prepare the sample of Comparative Example 5.
Example 5
Raw materials MnO.sub.2, NiO and Al(OH).sub.3 were ground and mixed
so that the molar ratio of the elements was Ni/Mn/Al=0.5/1.48/0.02,
and the mixture was calcined at 950.degree. C. for 8 hours. The
obtained NiMnAl composite oxide and Li.sub.2CO.sub.3 were mixed so
that the molar ratio of Li, Mn, Ni and Al was
Li/Ni/Mn/Al=1/0.5/1.48/0.02, and calcined in oxygen at 600.degree.
C. for 48 hours. Acetate salts of lithium, manganese, nickel and
aluminum were mixed so that Li/Ni/Mn/Al=1/0.5/1.48/0.02 in molar
ratio, and were dissolved in an aqueous solution. This solution and
positive electrode active material prepared above were mixed so
that the ratio of moles of Li+Ni+Mn+Al in the solution to moles of
Li+Ni+Mn+Al of the active material particles was adjusted to be
1/9. After mixing, the mixture was dried in an oven at 80.degree.
C., and calcined in air at 700.degree. C. for 8 hours. The crystal
structure was evaluated in the same manner as for positive
electrode active material 1, and a battery was manufactured in the
same manner as in Comparative Example 1 to prepare the sample of
Example 5.
Comparative Example 6
Raw materials Li.sub.2CO.sub.3, MnO.sub.2, NiO and Mg(OH).sub.2
were mixed so that the molar ratio of Li, Mn, Ni, Mg was
Li/Ni/Mn/Mg=1/0.5/1.48/0.02, and the mixture was calcined at
850.degree. C. for 8 hours. The crystal structure was evaluated in
the same manner as for positive electrode active material 1, and a
battery was manufactured in the same manner as in Comparative
Example 1 to prepare the sample of Comparative Example 6.
Example 6
Raw materials MnO.sub.2, NiO and Mg(OH).sub.2 were ground and mixed
so that the molar ratio of the elements was Ni/Mn/Mg=0.5/1.48/0.02,
and the mixture was calcined at 950.degree. C. for 8 hours. The
obtained NiMnMg composite oxide and Li.sub.2CO.sub.3 were mixed so
that the molar ratio of Li, Mn, Ni and Mg was
Li/Ni/Mn/Mg=1/0.5/1.48/0.02, and calcined in oxygen at 600.degree.
C. for 48 hours. Acetate salts of lithium, manganese, nickel and
magnesium were mixed so that Li/Ni/Mn/Mg=1/0.5/1.48/0.02 in molar
ratio, and were dissolved in an aqueous solution. This solution and
positive electrode active material prepared above were mixed so
that the ratio of moles of Li+Ni+Mn+Mg in the solution to moles of
Li+Ni+Mn+Mg of the active material particles was adjusted to be
1/9. After mixing, the mixture was dried in an oven at 80.degree.
C., and calcined in air at 700.degree. C. for 8 hours. The crystal
structure was evaluated in the same manner as for positive
electrode active material 1, and a battery was manufactured in the
same manner as in Comparative Example 1 to prepare the sample of
Example 6.
Comparative Example 7
Raw materials Li.sub.2CO.sub.3, MnO.sub.2, NiO and Co.sub.3O.sub.4
were mixed so that the molar ratio of Li, Mn, Ni, Co was
Li/Ni/Co/Mn=1/0.48/0.02/1.5, and the mixture was calcined at
850.degree. C. for 8 hours. The crystal structure was evaluated in
the same manner as for positive electrode active material 1, and a
battery was manufactured in the same manner as in Comparative
Example 1 to prepare the sample of Comparative Example 7.
Example 7
Raw materials MnO.sub.2, NiO and Co.sub.3O.sub.4 were ground and
mixed so that the molar ratio of the elements was
Ni/Co/Mn=0.48/0.02/1.5, and the mixture was calcined at 950.degree.
C. for 8 hours. The obtained NiMnCo composite oxide and
Li.sub.2CO.sub.3 were mixed so that the molar ratio of Li, Mn, Ni
and Co was Li/Ni/Co/Mn=1/0.48/0.02/1.5, and calcined in oxygen at
600.degree. C. for 48 hours. Acetate salts of lithium, manganese,
nickel and cobalt were mixed so that Li/Ni/Co/Mn=1/0.48/0.02/1.5 in
molar ratio, and were dissolved in an aqueous solution. This
solution and positive electrode active material prepared above were
mixed so that the ratio of moles of Li+Ni+Mn+Co in the solution to
moles of Li+Ni+Mn+Co of the active material particles was adjusted
to be 1/9. After mixing, the mixture was dried in an oven at
80.degree. C., and calcined in air at 700.degree. C. for 8 hours.
The crystal structure was evaluated in the same manner as for
positive electrode active material 1, and a battery was
manufactured in the same manner as in Comparative Example 1 to
prepare the sample of Example 7.
Table 3 shows the results of evaluation of crystal structures that
were measured by the same manner as for positive electrode active
material 1 and the result of evaluation of batteries that were
manufactured in the same manner as in Comparative Example 1. Since
they are assigned to Fd-3m or P4.sub.332 based on the crystal
structure evaluation, estimated compositions of crystals are
indicated in forms of LiM.sub.2O.sub.4 in Table 3.
TABLE-US-00003 TABLE 3 crystal structure Capacity active material
at internal crystal structure retention ratio composition at
internal active material portion of active at surface of after 200
cycles portion composition at surface material active material at
45.degree. C. Comp. Ex. 3 LiNi.sub.0.5Mn.sub.1.4Ti.sub.0.1O.sub.4
LiNi.sub.0.5Mn.sub.1.4- Ti.sub.0.1O.sub.4 P4.sub.332 P4.sub.332 64%
Comp. Ex. 4 LiNi.sub.0.5Mn.sub.1.4Ti.sub.0.1O.sub.4
LiNi.sub.0.5Mn.sub.1.4- Ti.sub.0.1O.sub.4 Fd-3m Fd-3m 63% Example 2
LiNi.sub.0.5Mn.sub.1.4Ti.sub.0.1O.sub.4 LiNi.sub.0.5Mn.sub.1.4Ti-
.sub.0.1O.sub.4 P4.sub.332 Fd-3m 78% Example 3
LiNi.sub.0.5Mn.sub.1.4Ti.sub.0.1O.sub.4 LiNi.sub.0.5Mn.sub.1.2Ti-
.sub.0.3O.sub.4 P4.sub.332 Fd-3m 82% Example 4
LiNi.sub.0.5Mn.sub.1.5O.sub.4 LiNi.sub.0.5Mn.sub.1.2Ti.sub.0.3O.-
sub.4 P4.sub.332 Fd-3m 80% Comp. Ex. 5
LiNi.sub.0.5Mn.sub.1.48Al.sub.0.02O.sub.4 LiNi.sub.0.5Mn.sub.1-
.48Al.sub.0.02O.sub.4 Fd-3m Fd-3m 62% Example5
LiNi.sub.0.5Mn.sub.1.48Al.sub.0.02O.sub.4 LiNi.sub.0.5Mn.sub.1.48-
Al.sub.0.02O.sub.4 P4.sub.332 Fd-3m 72% Comp. Ex. 6
LiNi.sub.0.5Mn.sub.1.48Mg.sub.0.02O.sub.4 LiNi.sub.0.5Mn.sub.1-
.48Mg.sub.0.02O.sub.4 Fd-3m Fd-3m 63% Example 6
LiNi.sub.0.5Mn.sub.1.48Mg.sub.0.02O.sub.4 LiNi.sub.0.5Mn.sub.1.4-
8Mg.sub.0.02O.sub.4 P4.sub.332 Fd-3m 73% Comp. Ex. 7
LiNi.sub.0.48Co.sub.0.02Mn.sub.1.5O.sub.4 LiNi.sub.0.48Co.sub.-
0.02Mn.sub.1.5O.sub.4 Fd-3m Fd-3m 57% Example7
LiNi.sub.0.48Co.sub.0.02Mn.sub.1.5O.sub.4 LiNi.sub.0.48Co.sub.0.0-
2Mn.sub.1.5O.sub.4 P4.sub.332 Fd-3m 70%
As similar to Table 2, the retention ratio is high in the case that
the surface of the active material has Fd-3m and internal portion
has P4.sub.332. It is considered that the same effect has been
obtained.
As described above, the constitution of the present embodiment
provides an effect of improvement in lifetime. Particularly, it is
highly effective in the case of using a positive electrode material
operable at potential of 4.5V or higher vs. lithium. Therefore, it
is possible to provide a lithium secondary battery having long
lifetime with a high operating voltage.
REFERENCE SIGNS LIST
1 positive electrode active material layer 2 negative electrode
active material layer 3 positive electrode current collector 4
negative electrode current collector 5 separator 6 laminate package
7 laminate package 8 negative electrode tab 9 positive electrode
tab
* * * * *